The GMOS-Train Metrology training course (29 – 30 Sept 2022) was hosted virtually by the Jožef Stefan Institute to provide basic concepts and knowledge about measurement uncertainty and traceability in chemical analyses. Dr. Igor Živković and prof. dr. Milena Horvat organized a series of lectures for all early-stage researchers (ESRs) of the Marie Skłodowska Curie ITN so they would be able to calculate the measurement uncertainties of their own mercury analysis in the environment. Following the training course, ESRs had regular video-chat consultations with the lecturers to help them calculate the measurement uncertainties of their analytical methods and to determine the main contribution components of their uncertainties. Their results were presented to an audience of about thirty participants at the GMOS-Train annual meeting (6 – 8 Mar 2023) at the ZMAW in Hamburg.